This invention broadly relates to the removal of mineral particles from other material and, more particularly, to an apparatus for electrostatically increasing the concentration of particles of precious metals in other material prior to a final separation step.
As the value of precious metals such as gold and silver continues to increase sources of these metals which in the past were not considered economically attractive for processing are now receiving closer attention. These sources include ore deposits as well as tailing or refuse dumps of previous removal operations.
Various factors such as, for example, the availability of ore with small mineral particles in relatively low concentrations and a greater concern about environmental pollution have resulted in a re-evaluation of the three most popular methods for removing gold of flotation, amalgamation and cyanidation. These three methods all use chemical reagents and were popular because they were at one time considered to be economical. However, now that environmental laws require that most contaminants be removed before dirt is returned to the original site, electrostatic separation for precious metals is being more seriously considered.
Since the particles to be removed are very small (gold particles, for example, have irregular shapes and range from less than a micron to greater than 0.1 inch in their largest dimension) the object of an initial processing step such as the one to which the present invention is directed is to remove a large amount of the non-precious material including the larger pieces so that the material which remains contains the precious metal in a relatively high concentration.
Although electrostatic precipitators have been used for separating or concentrating metal particles, in most prior art electrostatic mineral concentrators a collector electrode was either positively or negatively charged instead of having parallel collectors with different polarities. Such devices are the subject of U.S. Pat. Nos. 813,063; 2,270,526; 2,361,946; 3,012,668; 3,031,079 and 3,477,568. It was noticed in experiments with a parallel-plate electrostatic collector where a charge was placed on only one collector plate and the other plate was grounded and not charged as would normally be done, that the metal particles tended to collect on the grounded plate and in cracks and crevices around the grounded plate so that those particles were lost in the tailings.
However, even with electrostatic collectors where collector electrodes of different polarities are used such as in U.S. Pat. Nos. 856,711 and 3,720,312 maximum collection efficiency is not achieved because, it is believed, as the particles move through the concentrator they become attracted to one another, oftentimes neutralizing each other so that they are not collected and are lost in the tailings. It is also believed that as particles move through an electrostatic concentrator there are constant collisions between the particles, causing some of them to lose their charge and not be collected. When collecting precious metals such as gold it is obviously important to maximize the efficiency of the collection process.